I. Field of the Invention
The present invention relates to data communication. More particularly, the present invention relates to a novel and improved method and apparatus for measuring timing of signals received from multiple base stations in a CDMA communication system.
II. Description of the Related Art
A modern day communication system is required to support a variety of applications. One such communication system is a code division multiple access (CDMA) system that supports voice and data communication between users over a terrestrial link. The use of CDMA techniques in a multiple access communication system is disclosed in U.S. Pat. No. 4,901,307, entitled “SPREAD SPECTRUM MULTIPLE ACCESS COMMUNICATION SYSTEM USING SATELLITE OR TERRESTRIAL REPEATERS,” and U.S. Pat. No. 5,103,459, entitled “SYSTEM AND METHOD FOR GENERATING WAVEFORMS IN A CDMA CELLULAR TELEPHONE SYSTEM.” A specific CDMA system is disclosed in U.S. Pat. No. 6,574,211, entitled “METHOD AND APPARATUS FOR HIGH RATE PACKET DATA TRANSMISSION,” issued Jun. 3, 2003. These patents are assigned to the assignee of the present invention and incorporated herein by reference.
A CDMA system is typically designed to conform to one or more standards. Such standards include the “TIA/EIA/IS-95 Remote terminal-Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular System” (the IS-95 standard), the standard offered by a consortium named “3rd Generation Partnership Project” (3GPP) and embodied in a set of documents including Document Nos. 3G TS 25.211, 25.212, 25.213, 25.214, 25.133, 25.305, 25.331 and 3G TR 25.926 (the W-CDMA standard), and the “TR-45.5 Physical Layer Standard for cdma2000 Spread Spectrum Systems” (the cdma2000 standard). New CDMA standards are continually proposed and adopted for use. These CDMA standards are incorporated herein by reference.
A CDMA system can be operated to support voice and data communication. During a particular communication (e.g., a voice call), a remote terminal may be in active communication with one or more base stations, which are typically placed in an “active set” of the remote terminal. The remote terminal may also receive signals from one or more other base stations for other types of transmission such as, for example, pilot, paging, broadcast, and so on.
The CDMA system can be designed with the capability to determine the position of a remote terminal. In fact, the Federal Communications Commission (FCC) has mandated support for an enhanced emergency 911 (E-911) service whereby the location of a remote terminal in a 911 call is required to be sent to a Public Safety Answering Point (PSAP). For position determination, the arrival times of the transmissions from a number of base stations are measured at the remote terminal. The differences between the signal arrival times can then be computed and translated into pseudo-ranges, which are then used to determine the position of the remote terminal.
Various challenges are encountered in measuring the signal arrival times. For example, in a wireless communication system, the mobility of the remote terminal may affect the accuracy of the arrival time measurements, if these measurements are not made close in time. Also, the arrival times are typically measured based on the internal timing of the remote terminal, which may be continually adjusted to track the timing of one of the base stations with which the remote terminal is in communication. The remote terminal's movement and the variation (and uncertainty) in its time reference can impact the accuracy of the arrival time measurements, which may in turn translate to a less accurate estimate of the position of the remote terminal.
Accordingly, techniques that can be used to improve the accuracy of the arrival time measurements, which may lead to improved accuracy in the estimated position of the remote terminal, are highly desirable.